WASHINGTON — Months before Hillary Rodham Clinton started delivering Democratic primary voters a liberal-minded message about a “stacked deck” in favor of the wealthy and the need for criminal-justice reform, she met quietly at Esca, a Mario Batali restaurant in Midtown Manhattan, with four powerful labor leaders.

There, in February, she treated representatives of the biggest teachers’, service employees’ and government workers’ unions to a seafood dinner and a lengthy discussion of policy issues. Her meaning was unmistakable: She wanted them to feel like an important part of her coming campaign.

But Mrs. Clinton is not just conveying veiled messages to her party’s left-leaning base. She is also receiving them.

On Tuesday, Richard L. Trumka, president of the A.F.L.-C.I.O., gave an address here aimed ostensibly at any White House contender. Without naming Mrs. Clinton, he urged candidates for president to resist “cautious half-measures.”

He also called for “a commitment, from the candidate down through his or her economic team,” to steer a progressive agenda to completion. A senior labor official later called it the most important line in Mr. Trumka’s speech.

That is because he was publicly amplifying, however obliquely, an argument that liberals have made to Mrs. Clinton and her team more bluntly in private: They do not wish to see the likes of Robert E. Rubin and Lawrence H. Summers, both former treasury secretaries to Bill Clinton, become fixtures in her circle.

It is unclear if Mrs. Clinton will bow to such demands; a senior campaign aide said only that she receives a range of policy advice.

On Thursday, Senator Bernie Sanders of Vermont, a self-proclaimed socialist, said he would seek the Democratic nomination. But the push and pull between Mrs. Clinton’s pursuit of the liberal base and its desire for assurance before consummation may be a more significant source of tension within the party.

Mrs. Clinton cheerily welcomed Mr. Sanders into the race, even as she works to deny Mr. Sanders or any other liberal an issue on which to bloody her from the left.

She and her top aides have begun an aggressive charm offensive, calling, emailing, meeting and dining with scores of progressive officials and activists. John Deeth, a well-read blogger in Iowa City (“the People’s Republic of Johnson County,” he joked), said that Matt Paul, Mrs. Clinton’s Iowa director, called him after she announced.

“Mostly he listened,” recalled Mr. Deeth, who said he would probably remain neutral in the primary.

With labor opposing the Trans-Pacific Partnership trade pact, Mrs. Clinton’s staff has been in touch with Senator Sherrod Brown, Democrat of Ohio, who has been battling President Obama over the agreement.

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Senator Bernie Sanders of Vermont, right, a self-proclaimed socialist, is mounting a challenge to Ms. Clinton.CreditStephen Crowley/The New York Times

“People are seeing that the Democratic Party is again standing for things,” Mr. Brown said of the energy on the left. (He added that he was not ready to commit to Mrs. Clinton’s candidacy — but had “zero” interest in a presidential bid of his own.)

Liberal organizations say they are gearing up to mobilize their memberships around issues like campaign reform, trade and net neutrality. Their goal, leaders of some of the groups said, is to make clear to Mrs. Clinton that committing to their issues could be the difference between tens of thousands of additional enthusiastic volunteers going door to door for her and making contributions to her campaign or simply casting their ballot for her.

“The American people are pretty smart, and they’re used to being lied to in campaigns,” said Becky Bond, the political director of Credo, a progressive group that has raised about $76 million for liberal causes and has many members in states like Florida, Ohio and Pennsylvania. “They want some kind of reassurance that Hillary Clinton will back up her words with actions.”

The apprehensiveness is two-way. Mrs. Clinton’s campaign reached out through intermediaries to campaign finance activists, some of whom have been privately discussing a public effort in New Hampshire to pressure her on the issue. Campaign reform groups, in turn, have argued that she will be unable to persuade voters to back an activist government agenda unless she first addresses their concerns about Washington corruption.

Mrs. Clinton faces even more intense pressure to confront income inequality and make clear she is not beholden to the business-friendly policies preferred by some of her contributors. While aligning herself with the left on cultural issues like immigration and same-sex marriage, she has so far been less explicit on economic policies — avoiding taking a position, for example, on a controversial trade pact with Pacific Rim countries.

“No candidate can be all things to all people,” Mr. Trumka said in his speech, demanding that White House aspirants resist “the politics of hedged bets.”

That afternoon, Mrs. Clinton’s political director, Amanda Renteria, sent an email to leading union officials saying that the campaign had tapped an official from the United Food and Commercial Workers union as Mrs. Clinton’s “labor outreach director.”

“Filling this position has been a top priority for us given the importance of labor issues to Hillary and everyday Americans across the country,” Ms. Renteria wrote, noting Mrs. Clinton’s “longstanding relationships” with labor and her desire to nurture those ties “as a team.”

But a far weightier personnel matter is who will serve as Mrs. Clinton’s economic advisers, both on the campaign and, should she win, as president. Many on the left, who wish but are deeply skeptical that she will surround herself with advisers from the Elizabeth Warren school, believe both Mr. Obama and Mr. Clinton were overly captive to Wall Street-oriented economic aides.

Mrs. Clinton’s aides noted that she is taking economic advice from outspoken progressives like Gary Gensler, a former Wall Street regulator.

But Dean Baker, a liberal economist, said, “I would be very surprised if we had a President Clinton and the administration didn’t include the usual suspects.”

Mr. Baker, who has been frank about the need for a transaction tax on financial investments, said he was called by an adviser to Mrs. Clinton soliciting his input and ideas, but was unmoved by the approach.

“They’re being polite,” he said. “But I’m not optimistic. It may change if blood is drawn by Sanders or somebody. Maybe then they’ll have to sit down and talk seriously.”

A version of this article appears in print on , on Page A13 of the New York edition with the headline: Clinton Welcomes the Democratic Left, but Is Pressured to Take Progressive Stands . Order Reprints | Today’s Paper | Subscribe